I'm not sure subtlety is the key tbh. Someone who is ordinarily sensitive to others would not be lazy when they stayed in a house with 2 dds under 2.5. She might need it to be spelt out.
It's awkward but in my view better to lay out the ground rules VERY CLEARLY from the start (this comes from years and years of flat-sharing) and from having my extremely messy sister to stay fairly frequently.
Have a proper "meeting" with her around the dining room table tomorrow - agree dh definitely has to be on side - and say something like "it's great you are coming to stay, the dcs will love having their aunt around for so long, but it's non-stop here so if you don't mind ...."
and then give her a list of tasks and dos and don'ts. You are doing her a favour by letting her stay rent free after all. The least she can do is contribute a bit.
Also, I remember the days before I'd had a child and I genuinely didn't know HOW to help my sister when she'd had her first (although I didn't drop towels on her bathroom floor that's for sure). The routines involved in childcare can seem a bit bewildering and impenetrable if you haven't had experience of dc, so perhaps she would welcome some fairly firm guidance - it makes you feel like more of the family too - less of a "guest".
Get it straight from the start and you will save yourself loads of hassle later on ...
One thing I'd add to Ruby's list is to ask if she could try and fit in with the routine of the house as much as possible. When my sister stays she watches TV until 1 or 2 am every night (morning), wakes up everyone when she finally comes to bed, but then sleeps in late so that dd has been up for hours by the time she struggles downstairs, then she has breakfast when I'm trying to start lunch, and we clash at every turn, you get the picture, aaarrrgggghhhh!!!
Not sure I'd have the guts to say about asking her to leave if it doesn't work out though.
I think you are a very lovely s-in-law btw. Brownie points stacking up from your dh I hope...!!